1. Field of the Invention
The present invention relates to a light-receiving member, an image forming apparatus having the member, and an image forming method utilizing the member, and more particularly to a light-receiving member with excellent characteristics free from causing drawbacks such as a faint image or a smeared image regardless of the ambient conditions and without heating the light-receiving member, and capable of maintaining such characteristics, also an image forming apparatus having such light-receiving member and an image forming method utilizing such light-receiving member.
2. Related Background Art
For the element member employed for a light-receiving member such as an electrophotographic photosensitive member, there have been proposed various materials such as selenium, cadmium sulfide, zinc oxide, phthalocyanine, amorphous silicon (hereinafter abbreviated as a-Si), etc. Among these materials, non-single-crystal deposited films containing silicon atoms as the main component, as represented by a-Si, for example amorphous deposited films such as a-Si compensated with hydrogen and/or halogen (for example fluorine or chlorine), have been proposed as the photosensitive member of high performance, high durability and no ecological problem, and some deposited films have been practically used. U.S. Pat. No. 4,265,991 discloses an electrophotographic photosensitive member of which a photoconductive layer is mainly composed of a-Si.
The a-Si photosensitive member has a high surface hardness, a high sensitivity to the light of long wavelength region such as of a semiconductor laser (770 nm to 800 nm) and exhibits little deterioration even after repeated use, and is widely employed as the electrophotographic photosensitive member for high-speed copying machines and laser beam printers (LBP).
For forming such deposited films, there have been known various methods such as sputtering, thermal CVD, photo CVD, plasma CVD, etc. Among these methods, the plasma CVD in which a raw material gas is decomposed by glow discharge caused by a DC current, a high frequency (RF, VHF) or a microwave to form a thin deposited film on a substrate such as glass, quartz, a heat-resistant plastic film, stainless steel or aluminum has been particularly advanced, for example, for the formation of an amorphous silicon deposited film for practical use in electrophotography, and various apparatus have also been proposed for executing such formation.
Also in recent years, there have been made various considerations for meeting the increasing demands for an improved film quality and for a higher throughput.
In particular, the plasma process utilizing the high frequency power has been adopted because of various advantages such as stability of discharge and applicability in the formation of insulating films such as an oxide film and a nitride film. Also recently a report on the plasma CVD method employing a high frequency power source of 50 MHz or higher in a plasma CVD apparatus with parallel flat electrodes (Plasma Chemistry and Plasma Processing, Vol. 7, No. 3 (1987), pp. 267-373) has shown a possibility of improving the deposition rate, without deteriorating the properties of the deposited film, by elevating the discharge frequency beyond the conventionally employed frequency of 13.56 MHz. Such elevation in the discharge frequency has also been tried in sputtering processes and is being investigated widely.
In applying the a-Si photosensitive member, produced by such methods, to an image forming apparatus employing so-called electrophotographic technology, a corona charger (corotron or scorotron) is mostly employed as the charging and charge eliminating means for the photosensitive member. Such corona discharge generates ozone (O.sub.3) which oxidizes nitrogen in the air to generate corona discharge products such as nitrogen oxides (NO.sub.x), and thus generated nitrogen oxides etc. react with the moisture in the air to generate nitric acid or the like. Such corona discharge products, for example nitrogen oxides and nitric acid, are deposited on the photosensitive member and surrounding devices and contaminate the surfaces thereof. As the corona discharge products exhibit a low electrical resistance by moisture absorption, the charge retaining ability is substantially lowered over the entire area or in local areas, leading to image defects such as a faint image or a smeared image (due to deformation or no formation of the electrostatic latent image by the charge on the photosensitive member leaking along the surface thereof).
Also the corona discharge products deposited on the internal surface of a shield plate of the corona charger evaporate and are liberated not only while the image forming apparatus is in operation but also while the apparatus is stopped--for example, during the night. The evaporated products deposit on the surface of the photosensitive member corresponding to the aperture of the corona charger and absorb moisture, whereby the surface of the photosensitive member is reduced in electrical resistance. For this reason, the first copy (output) or several copies at the initial stage when the apparatus is started after a pause tend to show a faint image or a smeared image in an area which is opposed to the aperture of the corona charger while the apparatus is stopped. Such image smear, appearing like the trace of the charger, is often called the charger trace smear. Such defect becomes conspicuous when the corona charger is an AC corona charger.
The faint image and the smeared image induced by the corona discharge products become more serious when the photosensitive member is an a-Si photosensitive member. In comparison with other photosensitive members, the a-Si photosensitive member tends to exhibit a lower efficiency of charging and charge elimination, so that the charging and the charge elimination by corona discharge to the a-Si photosensitive member are conducted with a higher voltage for applying to the charger in order to significantly increase the charging current in comparison with the cases of other photosensitive members. Since the amount of ozone generation is proportional to the corona charging current, a configuration employing an a-Si photosensitive member in combination with a corona charger particularly generates a large amount of ozone, thereby eventually enhancing the faint image and the smeared image resulting from the corona discharge products. Also in the case of the a-Si photosensitive member, due to the adverse effect of the very high surface hardness thereof, the corona discharge products deposited thereon tend to remain firmly for a long time.
For preventing such faint image or smeared image, there have been contemplated the following two methods.
The first method consists of reducing the relative humidity by heating (30.degree. C. to 50.degree. C.) the surface of the photosensitive member by use of a heater incorporated into the photosensitive member or by blowing warm air to the photosensitive member by use of a warm air blower. This method is capable of evaporating the corona discharge products and the moisture deposited on the surface of the photosensitive member, thereby substantially avoiding the reduction in resistance of the photosensitive member surface.
The second method consists of increasing the water repellent property of the surface of the photosensitive member, thereby rendering the deposition of the corona discharge products more difficult and thus preventing the smeared image. For example, the Japanese Patent Application Laid-Open No. 61-289354 discloses an a-C surface layer subjected to plasma treatment with a fluorine-containing gas. Also the Japanese Patent Application Laid-Open No. 60-12554 discloses an electrophotographic photosensitive member having a surface layer composed of an amorphous material containing carbon and halogen atoms, and a producing method therefor. Furthermore, the Japanese Patent Application Laid-Open No. 63-65447 discloses the technology on a fluorine-containing organic polymer film defined by the relationship of an absorption coefficient of infrared absorption spectra, though it is principally intended for use as a charge transporting layer and has not been explained for use as a surface layer.
However, though the first method can solve the drawback of smeared image by the use of a heating device for the photosensitive member, it is preferable not to heat the photosensitive member by such heating device as a drum heater, in consideration of the energy saving and the ecology.
Also, when the a-Si drum of high image quality is adopted in a full-color copying machine and the photosensitive member is thus heated, the possibility of melt-adhesion, i.e., melting of toners to attach to the surface of the photosensitive drum, becomes higher since color toners are low-melting. Furthermore, the image density may locally become higher or lower at the interval of rotation of the cylindrical developer. Such fluctuation in the image density is induced by the expansion of the developer by the heat of the photosensitive member while the apparatus is stopped, and then the distance of the photosensitive member from an opposed portion thereof is reduced, thereby facilitating the transfer of the developer in comparison with the ordinary state. From these facts, there has been desired a photosensitive member which can avoid the faint image or the smeared image without heating.
On the other hand, with respect to the second method utilizing the improvement of the water repellent property, the aforementioned patent application describes the improvement of the water repellent property in case of exposure to ozone, but does not describe whether a durability test by a copying operation using a large number of papers has been practically conducted. The present inventors conducted a confirming test according to the method disclosed in the Japanese Patent Application Laid-Open No. 61-289354 and it proved to be an improvement on the smeared image in the initial period, but still showed smeared image after a continuous copying operation using a large number of papers.
A confirming test was also conducted on the method disclosed in the Japanese Patent Application Laid-Open No. 60-12554.
In this test it was proved that the fluorine-containing amorphous film or the organic polymer film was superior in preventing the smeared image from the initial period in comparison with the conventional surface layers and maintained such performance even after a continuous copying test.
However since the surface layer which is softer than the conventional surface layer gradually abraded by friction with paper and components arranged around the photosensitive member, the surface layer is required to have a higher hardness in order to maintain the performance of the surface layer up to the number of copying papers which is required for the a-Si surface layer with the ordinary thickness. It was also found that when a larger thickness was made larger in consideration of such abrasion, drawbacks such as an increased retentive potential and a lowered sensitivity were generated.
A confirming test was furthermore conducted on the method disclosed in the Japanese Patent Application Laid-Open No. 63-65447. In this prior technology, the physical properties are defined by the values of infrared absorption spectrum, but such definition is given in consideration of the properties for the charge transporting layer, and it proved to be insufficient in specific resistivity and in hardness for use as a surface layer.
In consideration of the foregoing, there is desired a photosensitive member (light-receiving member) provided with a surface layer of highly water-repellent property enough for preventing the faint image and the smeared image without heating, in which the water-repellent property is not deteriorated for a prolonged period, even after a copying operation using a large number of papers.
Also there is desired a technology capable of realizing high image quality in stable manner, in order to meet the recent requirement for the improvement of the copy image quality, in addition to the requirement for solving the drawback of smeared image. More specifically, a higher sensitivity and a thinner structure are requested for the light-receiving member (electrophotographic photosensitive member), in order to meet the various requirements, such as a higher definition, a higher operating speed, introduction of digital technologies, compactization, a lower cost etc. for the image forming apparatus such as the copying machine and the printer.
For meeting such requirements, the surface layer for protecting the surface of the photosensitive member is required to have a lower loss for the incident light and a thinner structure, but a thinner structure is in fact not practical in the conventional material for the surface layer. For this reason there is desired a novel material for the surface layer, having a wide band gap for realizing a low loss to the incident light and a high breakdown voltage, and allowing to form a thin film.